Tardiness pays off?

I signed up for a Consumer Behaviour research on "Potential Honours Students". I only signed up for it cause Rach told me to.

The exact scenario of how she told me to:
(I just came back from class)
Rach: So are you interested in pursuing honours after your degree?
Me: Urm...no..?
Rach: WRONG ANSWER! You're supposed to say yes.
Me: Yes...?
Rach: Good! So now you can sign up for the research participation and get $40 Co-op voucher.
Me: Oh cool!

So ya, I did. Then the lady kept confirming with me that I was attending, and I kept saying yes yes yes.

Then (it's today. Just 14 minutes ago to be exact), I was late for the thing. I was supposed to be there at 12.30pm. I reached at 12.35pm.

Then someone was standing at the door.
She was like are you here for the research?
I said ya.
Then she was like "Oh, I'm sorry. They actually already have enough people for it. I think they overestimated the number. So could you just sign here?", and then she handed me my $40 voucher.
So I was like, urm...so there's nothing for me to do?
She said, "No, I'm sorry. Thank you for your time though."

There you go. Tardiness does pay off sometimes.